The present invention relates, in general, to video signal processing in a high definition television (referred to hereinafter as HDTV), and more particularly, to an apparatus and a method for compressing/expanding a video signal.
2. Discussion of the Related Art
HDTVs have been regarded as the next-generation video communication means, and have received individual studies, which are classified as, for example, an HD-MAC system in Europe and a full digital ATV system in the U.S.A., since a study of a MUSE system of the analog transmission type was published by NHK, Japan, in the 1980s.
Generally, in pursuit of high picture quality and large-scale screen, such an HDTV has a resolution of at least twice that of an existing color TV, information amount of about five times that of the existing color TV, a screen size of at least thirty inches and a color reemergence of about ten times that of the existing color TV.
Also, the HDTV has a hi-fi sound quality corresponding to a compact disc. In an HDTV field, no local broadcasting station is required, and a satellite broadcasting system comes to the fore to completely clear up poor reception areas. Also, the satellite broadcasting system has been receiving careful study for combining with a ground broadcasting system.
In the HDTV studies up to the present, an HDTV signal has a bandwidth wider than existing television signals. Particularly in the U.S.A., it is prescribed that the HDTV signal be broadcasted simultaneously with an existing television signal having a bandwidth of 6 MHz. For this reason, the HDTV studies in and outside the country have been concentrated on a compression technique.
A general variable length coder uses a fixed codeword that is obtained by previously calculating a statistical value with respect to several video signals. Because the codeword used in the variable length coder is fixed, a bit amount is increased and performance is reduced in the case where an input video signal is not matched with a characteristic of the codeword. In order to solve this problem, means capable of using a codeword table with a plurality of codewords and sending a selected one of the codewords has been proposed. One example of this is shown in FIGS. 1A and 1B.
Referring to FIG. 1A, a block diagram of a compression part of a conventional apparatus is shown for compressing/expanding a video signal. The compression part of the conventional video signal compression/expansion apparatus is an encoder. As shown in FIG. 1A, the encoder includes a motion estimator 10 for estimating a motion of the video signal inputted through an input terminal Vin. A motion compensator 20 compensates for the motion of the video signal in response to a motion vector signal from the motion estimator 10. A subtracter 30 subtracts an output signal from the motion compensator 20 from the video signal inputted through the input terminal Vin to output a difference signal. A discrete cosine transformer (DCT) 40 performs a discrete cosine transform with respect to the difference signal from the subtracter 30 to remove a spatial redundancy. A quantizer 50 compresses output information from the discrete cosine transformer 40 so that the output information from the discrete cosine transformer 40 is approximated to a finite number of values, to reduce an amount of the output information from the discrete cosine transformer 40.
An inverse quantizer 60 is provided in the encoder to restore an output signal from the quantizer 50 to its original state. An inverse discrete cosine transformer 70 is also provided in the encoder to transform an output signal from the inverse quantizer 60 into a spatial signal.
An adder 80 is also provided in the encoder to add an output signal from the inverse discrete cosine transformer 70 to the output signal from the motion compensator 20 and feed-back the resultant signal to the motion compensator 20. A mapper 90 is provided in the encoder to output information corresponding to the output signal from the quantizer 50.
Also, the encoder includes a variable length coder (VLC) 100 for reducing an amount of the output information from the mapper 90 without losing the characteristic of the output information from the mapper 90. A buffer 110 prevents an underflow or an overflow of an output signal from the variable length coder 100 resulting from transmission at a fixed bit rate, and a formatter 120 formats an output signal from the buffer 110 to a suitable transmission format.
Referring to FIG. 1B, a block diagram of an expansion part of the conventional apparatus is shown for compressing/expanding the video signal. The expansion part of the conventional video signal compression/expansion apparatus is a decoder. As shown in FIG. 1B, the decoder includes a deformatter 130 for deformatting an output signal from the formatter 120 in the encoder in FIG. 1A to a suitable reception format. A buffer 140 adjusts the number of bits of an output signal from the deformatter 130 at a desired rate. A shifter 150 shifts an output signal from the buffer 140 and a programmable logic array 160 is connected to an output of the shifter 150. The programmable logic array 160 stores a predetermined codeword.
The decoder also includes a second mapper 170 for outputting information corresponding to the predetermined codeword in the programmable logic array 160.
A general vector quantization defines a K-dimensional Euclidean space R.sup.K as a map Q of finite partial sets Y, as follows:
Q: R.sup.K Y PA1 Y=/Xi: sets of reproduction vectors PA1 where, i=1, 2, . . . , N and N is the vector number of Y.
The vector quantization is generally classified into two functions, encoding and decoding. In the encoding function, the vector quantization maps an input vector X onto a reproduction vector address based on Q(X). In the decoding function, the vector quantization finds a reproduction vector /X based on the address in the encoding.
FIG. 2 is a view illustrating partitioning of a two-dimensional space into twenty-six cells. In this drawing, each round black dot means a representative value of each cell. A difference between the input vector X and the reproduction vector /X is called a distortion measurement d(X, /X). A simple example of the distortion measurement is a square error distortion that can be defined as follows: ##EQU1##
The vector quantization makes a set Y suitable for the input vector, compares the input vector with the set Y on the basis of the previously defined distortion to find a vector X with a minimum distortion, and sends an address of the found vector X.
Referring to FIG. 3, a block diagram of a general vector quantizer is shown. A codebook is produced suitably for the implementation of a system by means of a proper algorithm and then stored in codebook read only memories (ROMS) 3 and 3'. The encoder and decoder must have the same codebook.
Input data is rearranged in the same format as that of codewords in the codebook and then compared with all the codewords in the codebook according to a nearest neighbor rule 2. Then, a codeword with a minimum distortion is found among the codewords in the codebook based on a distortion measurement employed by the system. As a result, an address of the found codeword is sent. In this case, the distortion measurement used may include a mean square error, a weighted mean square error, and a linear estimate measurement.
Based on the sent codeword address, the decoder finds an original value of the codeword using a lookup table 4 and transmits the found value. As a result, a transmission rate is significantly reduced.
The conventional video signal compression/expansion apparatus has a disadvantage in that the codeword with the minimum distortion is selected among the codewords in the codebook by comparing the input data with all the codewords in the codebook. In other words, the search time for the codewords is increased relative to a size of the codebook. Also, the comparison of the input data with all the codewords is very inefficient and requires a separate high speed search device in hardware implementation.